It's fairly well-known that a house must have a well-insulated building envelope to be structurally sound. But we may not know how important this shell really is. You can decrease the amount of energy you use to maintain the interior climate of your house by minimizing the transfer of heat through this envelope.

Insulation can dramatically block the transfer of heat through this layer, as well as help control for sound. When insulating your home, it's advisable to get the R-value recommended by the Department of Energy or that specified by your area's local energy code, according to the North American Insulation Manufacturers Association. R-value is simply a measure of thermal resistance; the bigger the R-value, the more effective a home's insulation will be. To select high-quality insulation that will wear well for years in your geographic area, ask the local gas or electric utility company for their advice on good insulation for your location. In the following pages, we'll discuss some of the various forms of insulation they might recommend.

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Fiberglass Insulation

Fiberglass insulation is commonly found in air ducts, pipes, roofs, walls and floors. There are two types: fiberglass loose-fill insulation, which is blown in, and fiberglass blanket insulation, which comes in batts or rolls and in different densities, widths and lengths. Fiberglass is fibrous and made of sand and recycled glass, rendering it naturally noncombustible. If exposed to water, it won't retain moisture; instead, it will dry out and retain its original R-value. R-values for fiberglass insulation range from R-11 to R-38.

Fiberglass insulation can reduce energy use in homes and lower utility bills. This is why the North American Insulation Manufacturers Association considers it a cost-effective and energy-saving product. It has environmental benefits, too. Because it improves buildings' energy efficiency, fiberglass insulation helps reduce the amount of fossil fuel combustion needed to cool and heat buildings. The amount of carbon dioxide emitted into the atmosphere is thereby reduced.

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Cellulose Insulation

The insulation that keeps your house warm could be made from the newspaper you read.

Cellulose can be found in attics, walls, ceilings and other places around the home. It's mostly made of shredded newspaper, which makes it quite combustible. Although cellulose insulation is generously treated with fire retardant, it isn't fireproof. Cellulose installation also absorbs and retains moisture. If soaked, its thermal performance can be permanently diminished. Cellulose insulation can be applied a variety of ways. One method is to add water to it; however, this wet-spray method does not achieve full R-value until dry.

Insulation made of cellulose uses the same amount of virgin materials as that used to make fiberglass. However, it takes up to three times more cellulose than fiberglass to achieve the same insulating efficiency. Cellulose not only settles much more than other types of insulation (20 percent), but it does so at a higher rate. This is why the Insulation Contractors Association of America recommends adding 25 percent thickness to cellulose insulation to compensate for major R-value loss.

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Polyisocyanurate Insulation

Polyisocyanurate is a type of plastic made with closed-cell foam. Inside its cells is a low-conductivity gas, which gives the insulation an R-value ranging from R-5.6 to R-8. Gas can leak out of the cells over time and let air in, which can lower the R-value.

Polyisocyanurate insulation comes in several forms: liquid, sprayed foam and rigid foam board. It also can be made into laminated insulation panels with an array of different facings. Foam-in-place applications are often less expensive than installing foam boards, and they tend to perform better because the liquid foam molds to all surfaces.

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Expanded Polystyrene

Polystyrene is used to make concrete blocks. The expanded version can be incorporated into a type of insulation.

Polystyrene is a transparent, colorless thermoplastic. It's commonly used to make foam board, concrete block and loose-fill insulation.

Expanded polystyrene (EPS) is a specific type of polystyrene made of small plastic beads fused together. It's typically used in concrete blocks. In contrast, molded expanded polystyrene (MEPS) insulation is available as small foam beads. MEPS is commonly used for foam board insulation. Although it can easily be poured into hollow wall cavities, MEPS is notoriously challenging to control -- a small hole in the wall or a gust of wind will send the foam beads scattering all over. Foam board polystyrene's thermal resistance depends on its density and usually ranges from R-3.8 to R-5.0.

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Extruded Polystyrene

The manufacturing process differentiates extruded polystyrene (XPS) from expanded polystyrene (EPS), which we discussed on the previous page. XPS starts as a molten material that's pressed into sheets. It's usually used as foam board insulation, although it can be used for general or specialized applications. XPS is resistant to moisture, rot, mildew and corrosion. Facers or laminate aren't required for it to adequately resist water absorption. The Extruded Polystyrene Foam Association reports that when XPS is used in a house for 15 to 50 years, it has net positive energy conservation and air emission benefits (during this time more energy is saved than consumed by manufacturing the insulation).

Spray polyurethane foam is an insulating foam plastic that's sprayed on as a liquid that expands many times over. Thanks to specialized equipment, application techniques can achieve dramatically different results. The same raw materials, for example, can be made fairly rigid and soft to the touch. They can also be made into roofing foam that's resistant to foot traffic and water. Spray polyurethane foam doesn't just provide an air barrier and method of moisture control -- it also can offer high levels of R-value. Spray polyurethane foam can be used to insulate roofing, air barriers, walls, ceilings, attics and basements. It's effective at low and high temperatures and can provide a more ideal environment for your home's ventilation system so that it functions more efficiently.

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Radiant Barriers

A radiant barrier is a single reflective surface that faces an open space. Its purpose is to reduce summer heat gain and winter heat loss, and it's usually installed in attics. Radiant barriers are always installed with the reflective surface facing on open air space.

Here's how this type of insulation works: Radiant heat moves in a straight line away from surfaces, heating anything solid that absorbs the energy. So when the sun heats your roof, this happens mostly because of radiant energy. A lot of this heat travels through the roof to the inside attic side, where it's radiated onto anything cooler, like the attic floor. A radiant barrier minimizes radiant heat transfer from the underside of the roof to other attic surfaces. It can effectively block 90 to 97 percent of radiant heat striking its surface, which translates to lower energy costs.

However, keep in mind that radiant barriers don't provide much thermal insulation.

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Reflective Insulation

Cardboard isn't just for cereal boxes -- it can be used to make the backing for reflective insulation.

Reflective insulation is fabricated from some type of metallic foil, such as aluminum, and backed with plastic film, polyethylene bubbles, cardboard or kraft paper. The product is engineered to stop radiant heat transfer between open spaces. It's similar to radiant barriers but differs in that it includes a radiant barrier plus other insulation materials.

Reflective insulation is usually found between roof rafters, wall studs or floor joists. Its performance and cost-effectiveness over time depend on how the insulation is installed, where it's installed and how much existing insulation might be in a house. The materials reflective insulation is made of will conduct electricity, so hiring a professional to install it is advised.

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Cotton Insulation Made from Recycled Blue Jeans

Your jeans can serve a far greater purpose than being something to wear to work on casual Fridays. Made from post-industrial recycled cotton fabrics (such as blue jeans), cotton insulation earns kudos not only for its environmental and health safety, but also for its ability to dampen sound better than some other kinds of insulation. Installers aren't required to wear respiratory or safety equipment when dealing with cotton insulation, and no warning labels are required on the product. A nontoxic fire- and pest-repellant called Boron is used to treat it. Also, in colder climates, cotton insulation maintains its R-value. Cotton insulation comes in batts and can be installed in a similar way to that of fiberglass insulation.

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Mineral Wool

No matter what kind of insulation you choose for your home, the bottom line is that it's got to keep you cozy.

On average, mineral wool is made of 75 percent post-industrial recycled content. It comes in both blanket and loose-fill forms. Additional chemicals to make it fire-resistant aren't required, much like cotton insulation. Most often, mineral wool is a classification for two kinds of insulation: Rock wool is a man-made material that includes natural minerals, and slag wool is a man-made material that comes from scum called blast furnace slag that collects on molten metal.

There are hundreds of mineral wool companies in the United States; however, the U.S. Department of Energy notes that there is a Canadian company that produces a superior product.